Doping with impurities does what to Silicon?

  1. I want to know ... what happens to silicon when some impurity is added to it.
    Where does silicon goes??
     
  2. jcsd
  3. When we doped phosphorus in site of silicon then some silicon is replaced and phosphorus is added to the site . Now I am asking for that silicon atom ??
     
  4. Any one here to reply
     
  5. They don't go anywhere, but remain in the lattice. If the doping level is small, the extra silicon atoms just get absorbed by the lattice spacing getting slightly smaller. If the doping level is large, there are typically lattice defects that absorb the extra atoms. For this reason, heavily doped junctions are always leakier than lightly doped junctions.
     
  6. U mean that silicon atom is now free to move in the lattice? ?
     
  7. UltrafastPED

    UltrafastPED 1,916
    Science Advisor
    Gold Member

    No, neither the silicon atoms, nor the dopant atoms can move ... they simply make the crystal lattice "imperfect". I suppose an atom may move back and forth between two sites, especially with changes in temperature, but this is more a behavior of surface atoms which are weakly bound.

    The effect of the dopant is to change the conductivity of the silicon; it also makes it N or P type: a donor of negative (electrons) or positive (holes) charges ... as with the NP diode.

    In other words, it is a method of engineering the properties of the material: the dopant and the density both change the properties.
     
  8. The usual method to introduce impurities in silicon processing is with ion implantation, where the dopant atoms are shot into the silicon crystal at high energies. If you just do the ion implantation and look at the electrical properties, most of the dopant atoms are not electrically active, since they are sitting at interstitial sites (between the silicon atoms) instead of at lattice sites. For this reason, one always follows an ion implantation with some sort of high temperature anneal, typically at least 600C. During the anneal, the dopant atoms have enough energy to move about in the lattice, and they migrate from interstitial sites to lattice sites. They then become electrically active. There is also some movement of the silicon atoms during this anneal, and lattice defects created by the ion implantation process are generally repaired, since the uniform lattice is a lower energy state. The extra atoms are taken up in the lattice by just a slight change in the lattice spacing, as I said earlier, which introduces some lattice strain. However, if there are a large number of dopant atoms, the strain gets to be too large for the lattice to sustain, and defects appear.
     
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